P2404 on 2023 Subaru Legacy: EVAP Pump Circuit Fault and ECM Update Guide

What's Unique About the 2023-2023 Subaru LEGACY

For this specific model and year, Subaru has officially acknowledged that the original ECM software is overly sensitive and can falsely trigger a P2404 code. A Technical Service Bulletin (TSB #09-108-23) was issued to fix this with a software update. The bulletin explicitly states that "In rare cases, the internal pump for the Evaporative Leak Check Module (ELCM) system can seize temporarily. The new files contain enhanced logic to prevent the ELCM pump from seizing." Therefore, the first and most critical step is a software check, not hardware replacement. This issue is common across many modern Subaru models built on the Subaru Global Platform.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light is on
• Disabling of safety systems like EyeSight and Reverse Automatic Braking (RAB)
• Disabling of other features like automatic high beams or SI-Drive
• Multiple warning lights on the dashboard, often described by owners as a 'Christmas tree'

Most Likely Causes

1. Outdated Engine Control Module (ECM) Software 🔴 High Probability Subaru has officially acknowledged this issue through TSB #09-108-23. The original software logic is prone to misinterpreting signals from the leak detection pump, especially if it temporarily seizes due to moisture or cold weather. The updated software includes 'enhanced logic to prevent the ELCM pump from seizing'.
   How to confirm: A Subaru dealership or a shop with a Subaru Select Monitor (SSM) can check the current software calibration ID (CID) against the updated versions listed in TSB #09-108-23. This service is typically covered under the federal emissions warranty (8 years/80,000 miles).
   Typical fix: Reprogramming the ECM with the latest software update provided by Subaru. This is a software-only fix and may be covered under the emissions warranty.
   Est. part cost: $0
2. Faulty EVAP Leak Detection Pump (ELCM) 🟡 Medium Probability If the software update does not resolve the issue, the pump itself may have failed. The internal sensor can fail, or the pump motor can wear out or seize. Forum users across multiple Subaru models (Forester, Crosstrek, Ascent) report this part being on national backorder, indicating a high failure rate and potential supply chain issues.
   How to confirm: After confirming the ECM software is current, a technician follows the diagnostic flowchart in TSB #09-108-23, which involves using a scan tool to command the 'Evaporative System Leak Test' and checking if the pump achieves the specified vacuum value (lower than -0.26 PSI).
   Typical fix: Replace the EVAP Leak Detection Pump, also called an Evaporative Leak Check Module (ELCM).
   Est. part cost: $150-$300
3. Wiring or Connector Issue ⚪ Low Probability The wiring harness to the pump is located underneath the vehicle near the charcoal canister, making it susceptible to corrosion from road salt, moisture, or rodent damage.
   How to confirm: Visually inspect the wiring harness and connector for damage, fraying, or corrosion. Use a multimeter to check for proper voltage and continuity from the ECM to the pump connector pins. A resistance reading of 1 Ohm or higher suggests a problem in the harness.
   Typical fix: Repair the damaged section of wire or replace the corroded connector pigtail.
   Est. part cost: $20-$100

Diagnosis Steps

1. Crucial First Step: Check with a Subaru dealer or qualified shop if the vehicle's ECM has the latest software. Reference TSB #09-108-23.
2. If an update is available, have the ECM reprogrammed. The dealer will use a Subaru Select Monitor (SSM) and a stable power supply (13.5V) for this process. This resolves the majority of cases.
3. If the code returns after the update, or if the software is already current, proceed with hardware diagnosis per the TSB.
4. Locate the EVAP leak detection pump (ELCM), which is typically near the charcoal canister at the rear of the vehicle, behind the fuel tank.
🎬 See this walkthrough for accessing and replacing the leak detection pump.
5. Inspect the electrical connector and wiring harness for any obvious damage, corrosion, or loose connections.
6. Using a scan tool (like the Subaru Select Monitor), command the 'Evaporative System Leak Test' to run.
7. Observe the vacuum value results. The TSB specifies that the value in MID: $3C, TID: $C4 🎬 Watch: Real-world diagnosis of P2404 on a Subaru Global Platform vehicle. should be lower than -0.26 PSI. If it is not, or if the pump fails the active test, the pump has likely failed and needs to be replaced.

Parts You'll Likely Need

• EVAP Leak Detection Pump (Evaporative Leak Check Module - ELCM) (OEM #42087AN00A (Verify with dealer VIN)) — If the ECM software update does not fix the code, the pump itself is the next most likely component to have failed due to an internal sensor or motor fault. This part is frequently reported by owners and technicians to be on backorder, suggesting a high failure rate.
  Trusted brands: Subaru (OEM)
  OEM price range: $200-$300
  Aftermarket price range: $150-$250

Technical Service Bulletins (TSBs) & Recalls

• 09-108-23: The primary TSB for this vehicle. Announces diagnostic procedures and new ECM reprogramming files to address DTC P2404 on 2020-23 Legacy/Outback models. States the new logic prevents the pump from temporarily seizing.
• 09-122-24R: A broader TSB that also addresses P2404 with an ECM update, but applies to 2020-22 Legacy/Outback 2.5L, as well as Forester, Impreza, and Crosstrek models, showing the widespread nature of this software issue.
• 09-109-23: A similar TSB for older 2016-19 Legacy/Outback models, indicating a history of this problem. This update incorporated a 'cleaning mode' for the pump.

Platform-Specific Known Issues

• Subaru has issued TSB #09-108-23 specifically for the 2020-23 Legacy/Outback to address false P2404 codes via an ECM software update. The bulletin notes the new software contains 'enhanced logic to prevent the ELCM pump from seizing'.
• Owners on forums like Reddit frequently report this exact issue on new Subarus, often in cold weather, confirming the symptoms of disabled EyeSight and the fix being either the TSB or a backordered pump.

Mechanic-Grade Diagnostic Values

• ELCM Vacuum Value during 'Evaporative System Leak Test' — expected: Value should be lower than -0.26 PSI.. Failure: A value of -0.26 PSI or higher, or if DTC P2404 is stored again immediately after the test.
• Wiring Harness Continuity — expected: Less than 1.0 Ohm.. Failure: Resistance of 1.0 Ohm or higher indicates excessive resistance in the circuit.
• Power Supply Voltage for ECM Reprogramming — expected: A steady 13.5 Volts.. Failure: Voltage rising above 14.0 Volts can cause the reprogramming procedure to abort.
• ELCM Pump Sense Circuit Voltage (Pump OFF) — expected: Approximately 10 Volts.. Failure: Voltage significantly different from 10V on the ECM side of the circuit when the pump is commanded off.

Hidden / Shadow Codes Worth Checking

• MID: $3C, TID: $C4: These are Mode 6 Test IDs used by the Subaru diagnostic routine. They specifically report the vacuum value achieved by the ELCM during the 'Evaporative System Leak Test'. (see via This data is viewed using a Subaru Select Monitor (SSM) or an advanced scan tool capable of reading manufacturer-specific Mode 6 data after running the EVAP self-test.)

Scan Tool Commands That Help

• Subaru Select Monitor (SSM): Evaporative System Leak Test — This is the primary diagnostic function to run after confirming the ECM software is up-to-date. It commands the ELCM to run its self-test and reports the resulting vacuum level to confirm if the pump is functioning correctly.
• Subaru Select Monitor (SSM): ELCM Pump Active Test — This is a more intensive test, used if the standard leak test is inconclusive or the fault is intermittent. It runs the pump continuously for an extended period (e.g., 35+ minutes) to see if it fails under load or to free a temporarily seized motor.

Wiring & Ground Locations

• ELCM (Evaporative Leak Check Module) — At the rear of the vehicle, located behind the fuel tank and typically mounted on or near the charcoal canister assembly.. This is the location of the pump and its primary electrical connector, which is the focus of P2404. Physical inspection for corrosion or damage is a key diagnostic step.
• ELCM Connector — On the ELCM pump assembly at the rear of the vehicle.. Technicians can back-probe this connector to test for power, ground, and the sense signal. For example, on a similar model, Pin 3 (Red wire) and Pin 4 (Gray wire) were used to monitor the pump's voltage and ECM control with an oscilloscope.

Real Owner Repair Stories

• Reddit r/SubaruForester user Round-Geologist-3692 (2023 Forester Premium, 13,000 miles) — Recurring Check Engine Light with code P2404, along with disabled EyeSight/RAB.
  ❌ Tried (didn't work) Tightening the gas cap (initial dealer misdiagnosis)., A system 'reboot' performed by the dealer.
  ✅ What actually fixed it The dealer kept the vehicle, re-ran the EVAP test which then failed, and identified a malfunctioning 'evap check valve' (the ELCM assembly). The part was replaced under warranty to resolve the issue.

"I Checked Everything" — The Actual Cause

• A standard EVAP smoke test will almost always pass and show no leaks for a P2404 code. This is because the code indicates an electrical 'Sense Circuit Range/Performance' issue, not a physical leak that a smoke test is designed to find. Technicians can waste significant time searching for a leak when the root cause is either the ECM software logic being too sensitive or an internal electrical failure of the pump module itself, both of which are addressed by following Subaru's TSB #09-108-23.